Monolithically integrated tunable laser diodes, and in particular widely tunable laser diodes, are considered key devices for future High-Density Wavelength Division Multiplexing (HD-WDM) communication systems. When used as a spare transmitter, they can replace a large number of fixed wavelength Distributed Feedback (DFB) laser diodes and thus reduce inventory costs significantly. In addition, they can be used to introduce additional functionality into optical networks and thus can improve network flexibility or reduce the complexity of other network elements such as cross connects. Dynamic switching of the transmitter wavelength allows to some extent the introduction of packet switching. The tunable laser is also an essential part of another future key component of optical networks, the wavelength converter with tunable pump wavelength.
All widely tunable laser diodes based on electric tuning (i.e., with a tuning range of at least a few tens of nanometers) presented so far rely on at least 3 tuning currents, e.g., a current for each of the 2 grating sections and a phase current in the case of Distributed Bragg Reflector (DBR)-type devices. M.-C. Amann, J. Buus, “Tunable laser diodes”, Artech House, Norwood, Mass., 1998. The presence of at least 3 tuning sections has a number of serious drawbacks. First, the entire characterization of the laser and the selection of stable operation points require an extensive, time-consuming and costly characterization (in a 3-dimensional space if the active section current is not considered). Second, the long cavities that result from the presence of at least 3 (often passive) tuning sections in series prohibit direct modulation at high modulation frequencies and often limit the maximum output power.
Tunable twin guide (TTG) laser diode [M.-C. Amann, S. Illek, C. Schanen, et al., “Tunable Twin-Guide Laser—A novel laser diode with improved tuning performance”, Appl. Phys. Lett., Vol. 54, pp. 2532-2535, 1989.], presented as an alternative for the long 3-section DBR laser diodes, has a single tuning possibility; hence, tuning is based on a single tuning current, and the tuning of said TTG laser diode is limited.
Over the past years, several advanced laser structures have been proposed with an extended tuning range. Examples are the Y-laser [M. Kuznetsov, P. Verlangieri, A. G. Dentai, C. H. Joyner, and C. A. Burrus, “Design of widely tunable semiconductor three-branch lasers,” J. Lightwave Technol., vol. 12, no. 12, pp. 2100-2106, 1994], the co-directionally coupled twin-guide laser [M.-C. Amann, and S. Illek, “Tunable laser diodes utilizing transverse tuning scheme,” J. Lightwave Technol., vol. 11, no. 7, pp. 1168-1182, 1993], the Sampled Grating (SG) DBR laser [V. Jayaraman, Z. M. Chuang, and L. A. Coldren, “Theory, design and performance of extended tuning range semiconductor lasers with sampled gratings,” IEEE J. Quantum Electron., vol. 29, no. 6, pp. 1824-1834, 1993], the Super Structure Grating (SSG) DBR laser [H. Ishii, H. Tanobe, F. Kano, Y. Tohmori, Y. Kondo, and Y. Yoshikuni, “Quasicontinuous wavelength tuning in super-structure-grating (SSG) DBR lasers,” IEEE J. Quantum Electron., vol. 32, no. 3, pp. 433-440, 1996] and the Grating assisted Coupler with rear Sampled Reflector (GCSR) laser [M. Öberg, S. Nilsson, K. Streubel, L. Bäackbom, and T. Klinga, “74 nm wavelength tuning range of an InGaAsP/InP vertical grating assisted codirectional coupler laser with rear sampled grating reflector,” IEEE Photon. Technol. Lett., vol. 5, no. 7, pp. 735-738, 1993]. In the first two types of devices, a trade-off had to be made between the tuning range and the spectral purity (broad tuning range vs. high Side Mode Suppression Ratio (SMSR)). Therefore, recently most research attention has directed at the (S)SG-DBR and GCSR lasers.